Supercritical water oxidation (SCWO) is the oxidation of waste material in a body of water under conditions above the thermodynamic critical point of water, which is 374.degree. C. and 221 bar (2.21.times.10.sup.7 pascals). Water at supercritical conditions has a density of about one-tenth that of liquid water, and exhibits properties similar to those of a dense gas, including miscibility with organics and combustion gases in all proportions. These qualities result in a single-phase reaction medium containing water, organics and oxidizer(s), allowing oxidation to proceed without the need for mass transport across an interface. As a single-phase reaction medium, supercritical water provides an unusually high level of efficiency in oxidizing organics to harmless oxidation products. Operation under these conditions combines the benefits of high density, which permits high reaction rates at moderate temperatures, with high mass diffusivity and low viscosity. Air pollution problems are minimal compared with thermal incineration, since the operating temperature in a supercritical medium is low enough to prevent any significant formation of NO.sub.x or SO.sub.2. Furthermore, any acid gases formed by the oxidation can be neutralized in situ to inorganic salts by the addition of caustic. Since inorganic salts are not soluble in the supercritical water, they precipitate out, which facilitates their removal. When operated effectively, SCWO is applicable to waste water streams containing up to 20% organics by weight, with removal efficiencies in excess of 99.99% at typical reactor conditions of 500.degree. to 650.degree. C. and 250 bar (2.50.times.10.sup.7 pascals) with residence times of 10 to 30 seconds.
Temperature control is important in maintaining the desired reaction rate in SCWO and in preventing NO.sub.x and SO.sub.2 formation. Known methods of achieving this include controlling the heating value of the aqueous feed stream, either by diluting the stream with water or adding fuel. Preheating of the feed stream and cooling the reactor are also effective methods of controlling the temperature profile along the reaction flow path.
Two problems which arise in SCWO reactor configurations are the plugging of reactors due to the deposition of inorganic salts on the reactor walls, and corrosion of the reactor walls due to an imbalance in the neutralization. Plugging can be avoided by periodic shutdowns for cleaning, but as in all shutdowns, this considerably lowers the overall operational efficiency. Corrosion can be avoided by the use of corrosion-resistant materials, but these are expensive and in some cases difficult to form into reaction vessels of the desired configurations.